Could somebody please explain the strat for guaranteeing a slugpup spawn in Outskirts in the shelter near the gate to Industrial?

Starting as Survivor, get enough food to hibernate, and then make it into the first shelter after the starting room and hibernate at precisely 1:22 on the in-game timer. Milliseconds don't matter. If performed properly, this guarantees a pup will spawn in SU_S04 (usually the second shelter you'll find in Outskirts).

A note, sometimes if you are unlucky the slugpup can wander before you get to it, or it can be eaten by the pink lizard that often comes by that area. Its rare, but possible even when successfully manipulating the spawn.